Thermoplastic vulcanizates include blends of dynamically cured rubber and thermoplastic polymers. The rubber may be dispersed within the thermoplastic resin phase as finely-divided rubber particles. These compositions have advantageously demonstrated many of the properties of thermoset elastomers, yet they are processable as thermoplastics.
Thermoplastic vulcanizates may advantageously be prepared by dynamically vulcanizing a rubber with a curative agent while the rubber is being mixed with a thermoplastic resin. Factors that may influence the physical properties of a thermoplastic vulcanizate include those related to the physical properties of the rubber discontinuous phase such as the state of cure, the physical properties of the thermoplastic continuous phase, the size and the concentration of the particles of the discontinuous phase, the presence of fillers, curatives, and other additives present in the composition, and the like.
Physical properties of thermoplastic vulcanizates are often trade-offs between extremes. A need exists in the art for thermoplastic vulcanizate compositions having balanced properties. Ethylene based elastomers such as ethylene-propylene (alpha-olefin)-diene (EPDM) elastomers are generally polymers of very high molecular weight (as measured by their Mooney viscosity), and are often suitable for use in TPV applications. But, high molecular weight EPDM elastomers inherently possess very high viscosities, e.g., Mooney viscosity greater than 200 ML(1+4@125° C.). This inherent characteristic of EPDM results in difficulties related to the processability of these polymers. Extender oil is often added to the polymers to reduce the apparent viscosity.
The required level of extender oil depends on the molecular weight of the elastomer, but is usually sufficient to reduce the apparent viscosity of the oil extended EPDM to a Mooney viscosity of about 100 ML(1+4@125° C.) or below. Commercially available very high molecular weight EPDMs, which would be useful in TPVs, typically contain from about 50 to about 125 phr extender oil.
For example, EPDM as disclosed in WO 2000/26296 is directed to a metallocene based ethylene-alpha-olefin elastomeric composition made by a series reactor operation in which the high molecular weight component has a Mooney viscosity not exceeding 120, and is present in an amount no greater than 50 weight percent. Solvent utilized in this process is removed from metallocene based processes utilizing flash evaporation of the solvent, wherein reduced pressure is applied to the reaction product. However, at least a portion of the oil (if present) in the reaction product may become entrained in the solvent being removed under reduced pressure, and may be removed along with the solvent.
Metallocene based processes may thus be limited to a polymer product having an overall Mooney viscosity of less than about 90 ML (1+4@120° C.) in the absence of extender oil, due to the handling characteristics of such polymers including the difficulties of further processing polymers having a Mooney viscosity above about 90 ML (1+4@120° C.). However, polymer compositions having a Mooney viscosity of less than or equal to about 90 ML (1+4@120° C.) in the absence of extender oil have inferior properties.
WO 2003/066725A2 is directed to bimodal EPDM polymer compositions comprising a major polymer fraction having a Mooney viscosity above 120 ML(1+4@125° C.), and a minor polymer fraction having a Mooney viscosity of 120 ML(1+4@125° C.) or less, where the composition has a tan delta of 0.5 or less (125° C./10.4 rad/s). These compositions are essentially free of extender oil and preferably have an overall Mooney viscosity below 100 ML(1+4@125° C.) to ensure ease of processability. The compositions are prepared using metallocene catalysts in a series reactor process wherein the high molecular weight component is produced in the first reactor, and the low molecular weight component is produced in the second reactor, both using metallocene catalysts. Both components have relatively narrow molecular weight distributions with a polydispersity index (Mw/Mn) of less than 4, preferably less than 3. In addition, both components have relatively low branching as indicated by a relatively high average branching index of greater than 0.7, preferably greater than 0.8, on a scale from zero to one in which a branching index of 1 represents a linear polymer.